Estimation of effective aerodynamic roughness of Walnut Gulch watershed with laser altimeter measurements

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Abstract
A new method to estimate the effective aerodynamic roughness of a complex landscape is presented. High‐resolution elevation profiles measured with an airborne laser altimeter have been used to compute geometrical parameters of three landscape elements of the Walnut Gulch experimental watershed, Arizona. Mean crop height, its standard deviation, and the frequency distribution (as a function of penetration depth) of plant elements hit by the laser beam have been calculated for short segments (1 m) measured over intervening grass between shrubs. Longer segments have been applied to obtain the mean height and spacing of taller shrubs and trees. Finally, laser profiles covering the entire watershed gave the mean amplitude and wavelength of hillocks and ridges. The aerodynamic roughness length of the intervening grass is estimated using the ratio of standard deviation to vegetation height, corrected for instrument noise, times mean height. An effective roughness length which parameterizes the total stress due to grass and taller shrubs and trees is calculated first. Finally, a watershed‐scale effective roughness length is calculated combining the amplitude and wavelength of hillocks and ridges with the roughness length obtained in the previous step.